The present invention relates to a solid-state pickup element employing a vertical type overflow drain structure in, for example, a CCD solid-state pickup element, etc., and a method for producing the solid-state pickup element.
A solid-state pickup element of a so-called vertical type overflow drain structure, in which surplus charge is discharged to the substrate side at a light receptive sensor part, has been publicly known as a solid-state pickup element.
A solid-state pickup element has been proposed which has sensitivity in a near-infrared ray area by deeply forming a charge collecting area of a light receptive sensor part in a solid-state pickup element of a vertical type overflow drain system like this.
A general configurational view (cross-sectional view) of such a CCD solid-state pickup element having sensitivity in a near-infrared ray area is shown in FIG. 11.
The CCD solid-state pickup element 51 is such that the first semiconductor well area 53 of the second conductive type, that is, a p-type, which becomes an overflow barrier area, is formed on a semiconductor substrate 52 made of the first conductive type, that is, an n-type silicon, and a high resistance area 54 having high specific resistance such as, for example, pxe2x88x92area, non-doped area, nxe2x88x92area, etc., is formed on the semiconductor substrate 52 on which the first p-type semiconductor well area 53 is formed.
An n+ semiconductor area 56 that constitutes respective matrix-arrayed light receptive sensor parts 55 is formed on the surface of the high resistance area 54, and a p+ positive charge accumulating area 57 is also formed thereabove. The p+ positive charge accumulating area 57 prevents a dark current from occurring due to a phase boundary level. The n+ semiconductor area 56 becomes a so-called charge accumulating area. An nxe2x88x92 semiconductor area 70 having a denser concentration than that of the high resistance area, which will become a charge collecting area reaching the first p-type semiconductor well area, which is so-called overflow barrier area 53, from the n+ semiconductor area 56, is formed on the high resistance area 54 below the n+ semiconductor area 56. An area where photo-electric conversion of the light receptive sensor part 55 is carried out includes the n+ semiconductor area 56 and the nxe2x88x92 semiconductor area 70 before the overflow barrier area 53 of a depletion layer extending from the n+ semiconductor area 56 downward of the substrate.
An n-type embedded transfer channel area 60 of a vertical transfer register 59 is formed at the position corresponding to one side of the respective light receptive sensor part row of the high resistance area 54 so that a reading gate part 58 is placed between the same and the p+ positive charge accumulating area 57. Also, the second p-type semiconductor well area 61 is formed so as to surround the embedded transfer channel area 60. Further, a p-type channel stop area 62 is formed, which sections respective pixels including the light receptive sensor part 55.
A transfer electrode 65 made of, for example, polycrystalline silicon is formed on the embedded transfer channel area 60, channel stop area 62, and reading gate part 58 via a gate insulation film 64. The vertical transfer register 59 of a CCD structure is constructed of the embedded transfer channel area 60, gate insulation film 64, and transfer electrode 65.
A light shielding film 67 made of, for example, Al is formed on the entire surface excluding an opening of the light receptive sensor part 55 via an interlayered insulation film 66 that shields the transfer electrode 65. Light is made incident into the light receptive sensor part 55 via the opening of the light shielding film 67, and the light can be prevented from incidence into parts other than the light receptive sensor part 55 by the light shielding film 67.
Further, although not illustrated, a flattening film, a color film, on-chip lens, etc., are formed, thereby constituting a CCD solid-state pickup element 51.
And, in the above-described structure, as a method for improving the sensitivity with respect to a longer wavelength, as disclosed in, for example, Japanese Patent Laid-Open No. 331058/1997, a method has been proposed which deeply widens the charge collecting area 70 by causing a silicon epitaxial film of a low impurity concentration to thickly grow on the upper layer of the first p-type well area 53 as the high resistance area 54 of FIG. 11 in order to form an overflow barrier area at a deeper position.
Herein, in order to introduce electrons, which are photo-electrically converted at a deeper position from the surface, to the surface of the light receptive sensor part 55, it is necessary that the potential is inclined toward the surface side in a depletion layer between an n+ semiconductor area 56 that constitutes a photo diode of the light receptive sensor part 55 and a p-type semiconductor well area 53 that constitutes an overflow barrier.
Conventionally, the sensitivity with respect to a longer wavelength has been achieved by making the charge collecting area 70 into a low concentration area (nxe2x88x92).
Further, since, at the semiconductor area 54B downward of the second p+ semiconductor well area 61 that constitutes the vertical transfer register 59, inclination that is inverse of the light receptive sensor part 55 is formed to be potentially continuous with the first p+ type semiconductor well area 53, which is positioned further downward thereof, such a profile is provided, in which electrons flow in the direction of the substrate 52.
Therefore, as the pixel size becomes small, the low concentration area, that is, the charge collecting area 70 is subjected to three-dimensional potential modulation from the above-described semiconductor area 54B, whereby the overflow barrier will move to the surface side.
Resultantly, such a situation occurs, where no sensitivity is provided with respect to the longer wavelength side.
To suppress the situation, it is necessary to form the n+ semiconductor area 56 of the light receptive sensor part 55 further closer to the first p-type semiconductor well area 53.
Therefore, ion injection of high energy will be required.
However, in the present situation, there exists no equipment that can inject electrons with energy that is more than 3.2 Mev.
Therefore, where P (phosphor) is injected with energy of 3.2 MeV at maximum, the projection range Rp becomes Rp=2.6 xcexcm+xcex94Rp=0.3 xcexcm, wherein the n+ semiconductor area 56 can be formed at only a depth of 3 xcexcm in total.
Accordingly, the pixel size cannot be reduced further.
In order to solve the above-described problem, it is therefore an object of the invention to provide a solid-state pickup element that is capable of achieving improvement of the sensitivity and reducing the size thereof, and a method for producing the same.
A solid-state pickup element according to the invention is constructed so that an overflow barrier area is formed in a semiconductor substrate, an epitaxial layer is formed on the semiconductor substrate, a first conductive type semiconductor area, which widens a charge collecting area upward of the overflow barrier area, is formed so as to include at least the inside of the semiconductor substrate, and a charge accumulating area of a light receptive sensor part is formed at the position corresponding to the first conductive type semiconductor area of the epitaxial layer.
A method for producing the solid-state pickup element according to the invention includes the steps of forming an overflow barrier area in a semiconductor substrate, forming the first conductive type semiconductor area on the surface of the semiconductor substrate, forming an epitaxial layer on the semiconductor substrate, and forming the first conductive type charge accumulating area at the position corresponding to the first conductive type semiconductor area on the surface side of the epitaxial layer.
According to one of the aspects of the above-described solid-state pickup element of the invention, since the first conductive type semiconductor area is formed upward of the overflow barrier area, it will become possible to widen the charge collecting area to the vicinity of the overflow barrier area by the first conductive type semiconductor area. Since the first conductive type semiconductor area is formed so as to include at least the inside of the semiconductor substrate, it becomes possible to widen the charge collecting area to a deep position in the vicinity of the overflow barrier area formed in the semiconductor substrate.
And, since the charge accumulating area of the light receptive sensor part is formed at the position corresponding to the first conductive type semiconductor area, it is possible to accumulate charge photo-electrically converted by utilizing a deep charge collecting area in the light receptive sensor part.
According to the above-described method of the invention, since it includes the steps of forming the first conductive type semiconductor area on the surface of the semiconductor substrate and forming an epitaxial layer on the semiconductor substrate, the first conductive type semiconductor area can be formed at a deep position corresponding to the thickness of the epitaxial layer, wherein it is possible to construct a charge collecting area that is widened to a deep position.
And, since the first conductive type charge accumulating area is formed at the position corresponding to the first conductive type semiconductor area at the surface side of the epitaxial layer, it is possible to form a charge accumulating area in a deep charge collecting area.
According to the invention described above, it becomes possible to widen the charge collecting area close to the overflow barrier area by the first conductive type semiconductor area.
Thereby, since charge photo-electrically converted by utilizing a deep charge collecting area can be accumulated in the light receptive sensor part, conventionally wasted charge can be collected, and the sensitivity can be improved.
In addition, the thicker the epitaxial layer becomes, the deeper the first conductive type semiconductor area is formed, wherein the charge collecting area is widened, and satisfactory sensitivity can be brought about with respect to light of longer wavelength.
That is, it becomes possible to improve the sensitivity of light of a longer wavelength side and to improve the sensitivity of visible light.
Since the charge collecting area can be deepened without deeply forming the first conductive type semiconductor area of the light receptive sensor part, it is possible to prevent the overflow barrier from moving to the surface side in a case where the pixel size is made small.
Therefore, with the present invention, it is possible to improve the sensitivity of a solid-state pickup element and to reduce the pixel size.